[css-align-3] Clarify how baseline-sharing groups determine their fallback alignment. w3c#7645

Author: tabatkins

css-align-3/Overview.bs

@@ -2351,9 +2351,12 @@ Aligning Boxes by Baseline</h3>
 	the <a>alignment baseline</a> is the <a>dominant baseline</a> of the [=alignment context=].
 
 	Position the aligned <a>baseline-sharing group</a> within the [=alignment container=]
-	according to the <a>fallback alignment</a> associated with the
-	specified <a>baseline alignment preference</a>.
-	For first/last <a>baseline content-alignment</a>,
+	according to its <a>fallback alignment</a>.
+	The [=fallback alignment=] of a [=baseline-sharing group=]
+	is the [=fallback alignment=] of its items
+	as resolved to [=physical directions=].
+
+	For first/last <a>baseline <em>content</em>-alignment</a>,
 	then add the minimum necessary extra space
 	between the [=alignment container’s=] start/end content edge
 	and the [=alignment subject’s=] edge
@@ -2450,6 +2453,8 @@ Changes</h2>
 		(<a href="https://github.com/w3c/csswg-drafts/issues/5923">Issue 5923</a>)
 		<li>Better define how baseline self-alignment and content-alignment interact.
 		(<a href="https://github.com/w3c/csswg-drafts/issues/6389">Issue 6389</a>)
+		<li>Clarify how baseline-sharing groups determine their fallback alignment.
+		(<a href="https://github.com/w3c/csswg-drafts/issues/7645">Issue 7645</a>)
 	</ul>
 
 	Changes since the <a href="https://www.w3.org/TR/2018/WD-css-align-3-20181206/">6 December 2018 Working Draft</a> include: